[Doc]: Citation link in the readme.md points to 404

[Guyonsteroids]

Documentation Link

No response

Problem

The ready-made citation link at the bottom of the readme points to https://matplotlib.org/stable/project/citing.html, which says "Page not found"

Suggested improvement

In citing.html, add a premade BibTeX citation

[story645]

Oh, this is cause #27560 updated the url, but the PR was milestoned for 3.9

Page is at https://matplotlib.org/stable/users/project/citing.html

ETA: basically not sure if it's worth putting the link back til 3.9...